The screw apparatus includes a screw shaft and a nut that has a through-hole penetrated by the screw shaft and threadedly engages with the screw shaft via a rolling element. When the screw shaft is rotated, the nut moves in an axial direction of the screw shaft. The rolling element is interposed between the screw shaft and the nut. Accordingly, it is possible to eliminate backlash between the screw shaft and the nut and obtain light movement of the screw shaft and the nut.
When dust enters the inside of the nut, the smooth rolling motion of the rolling element is hindered. Hence, a seal ring that prevents the entry of foreign matter into the inside of the nut is attached to the nut. The seal ring has a hole that fits the screw shaft's shape of a cross-section perpendicular to the axis, and prevents the entry of foreign matter into the inside of the nut by bringing an inner periphery of the hole into contact with the screw shaft. A protrusion on the inner periphery of the seal ring is required to be fitted into a helical screw groove of the screw shaft. Accordingly, the seal ring is required to be attached to the nut with the seal ring positioned circumferentially with respect to the nut.
Patent Literature 1 discloses a known screw apparatus including a seal ring, which is configured in such manner as to include through-holes in seal rings and a cap, thread the through-holes with fastening members oriented in an axial direction of a screw shaft, fasten the fastening members to an end surface of a nut, and sandwich the seal rings between the cap and the nut. The screw apparatus described in Patent Literature 1 accurately performs circumferential positioning of the seal rings with respect to the nut by threading the fastening members through cylindrical sleeves and fitting the sleeves into the through-holes of the seal rings without any gaps.